Field of the Invention
The present invention relates to a method for electrolytically refining lead in a sulfamate bath and particularly to a method for electrolytically refining lead in a sulfamate bath which recovers lead contained in dry flue dust generated from nonferrous smelting, melting furnaces for recycle raw materials such as boards and electronic parts, and dry furnaces for the melting treatment of industrial waste.
Description of the Related Art
In order to recover lead contained in dry flue dust in nonferrous smelting from nonferrous smelting, melting furnaces for recycle raw materials such as boards and electronic parts, and dry furnaces for the melting treatment of industrial waste, the flue dust is subjected to sulfuric acid leaching to form lead sulfate, and then the lead sulfate is subjected to smelting reduction in an electric furnace. The metal separated by the smelting reduction is treated with soda, and then the metal is cast into an anode and then electrolytically refined in a sulfamate bath to recover lead.
As such an electrolytic refining technique for lead, for example, Japanese Patent No. 5163988 discloses a method for electrolyzing lead in which in electrolytic refining in a sulfamate bath, electrolysis is performed at a first-stage current density of 50 A/m2 or less for 2 hours or more, and then electrolysis is performed at 100 A/m2 or less as a second stage to recover high purity lead. It is described that according to such a configuration, also for a high Bi grade anode, high purity lead can be recovered.
Problems are that when the conventional electrolytic refining of lead in a sulfamate bath is carried out, a white residue is seen in the bath liquid, and further the lead concentration in the electrolytic solution decreases. In other words, the white residue is deposited at the bottom of the electrolytic cell, and when the white residue is deposited to some extent, it needs to be removed from the electrolytic cell, and for this purpose, the electrolysis is stopped. When the amount of the white residue is large, its frequency increases. In addition, when a decrease in the lead concentration in the electrolytic solution is seen, the electrodeposition condition of lead may worsen. The cause is that sulfamic acid during the electrolytic refining of lead in the sulfamate bath decomposes by the following reaction, and the white residue is deposited as lead sulfate, and the lead concentration in the electrolytic solution decreases.SO2NH2−+H2O→SO42−+NH4+Pb2++SO42−→PbSO4↓.